The invention concerns a composite material in band or slab form made of two cover plates, made of steel, which can be resistance welded together via electrically conductive spacer bodies, and an intermediate layer made of a filler material in which the spacer bodies are embedded.
The invention further concerns a process for the production of the composite material in band form and a facility for performing the process.
Composite materials of the type mentioned and processes for their production are known. These types of composite materials are, for example, used as deep-drawn molded components in motor vehicle construction because they have a high moment of inertia of the cross-sectional area in proportion to their weight, and therefore a high buckling stiffness, and are characterized by high oscillation damping and structure-borne sound damping properties.
The composite materials can be supplied as semifinished products in coil form or in slab form. The composite materials are, however, also sometimes further processed by the producers into finished products, for example through deep-drawing, reshaped, and then subjected to a cathodic immersion enameling with subsequent stoving of the enamel. This further processing into finished components can, of course, also be performed by the vehicle manufacturer.
In a known composite material of the type initially mentioned (DE 38 34 829 C2), the electrically conductive bodies consist of a ferrosilicon, stainless steel, or nickel powder, which is admixed into the filling material made of plastic which forms the intermediate layer. In this composite material, the powder, which has grain sizes of 70 to 130% of the thickness of the intermediate layer, exclusively serves for the purpose of allowing the composite material to be resistance welded, in that, during later roller seam or spot welding with a solid plate or another similar bonding plate, it produces the electrical connection between the welding electrodes. In contrast, the transverse strength between the cover plates is produced via the intermediate layer made of plastic, which adheres to the cover plates. Therefore, the intermediate layer has a thickness of a magnitude of approximately 1 mm. In these types of thin intermediate layers, it may also be possible to apply a mixture of plastic and powder whose grain size is up to 130% of the thickness of the intermediate layer.
The invention has as its object the development of a composite material of the type initially mentioned, a process for its production, and a facility for performing the process. The composite material is to have a very thick intermediate layer relative to the cover plates and should particularly be workable by deep-drawing. In this case, the cover plates should essentially provide the composite material with sufficient transverse stiffness and transverse strength.
According to the invention, the composite material of the type initially mentioned is characterized in that the electrically conductive spacer bodies, which are implemented as curved slugs, are pressed flat between the cover plates and are resistance welded to them.
In the composite material according to the invention, the cover plates are firmly bonded with one another via the welded-on slugs, so that the composite material has a high transverse strength of the cover plates relative to one another even as a semifinished product, i.e. before its further processing through reshaping. A further advantage of the composite material according to the invention is that the two cover plates remain essentially flat.
According to an embodiment of the invention, the filler material consists of an epoxy resin which polymerizes, particularly cross-links, only at enamel stoving temperatures. The use of this type of filler material has the advantage that a blank from the composite material can be reshaped into a molded component, particularly by deep-drawing, without problems, because the intermediate layer is also reshaped due to the epoxy resin, which has not yet been cross-linked, and the cover plates are not too greatly stressed and/or the cover plates do not come off of the intermediate layer. Simultaneously, it is also ensured that the intermediate layer is not pressed out from between the cover plates during reshaping due to its rigid state. Finally, for cathodic immersion enameling, to which the molded component is to be subjected, it is guaranteed that the epoxy resin does not melt and run out at the high, sustained temperatures typical for stoving, but polymerizes, thus only then producing the final fixed adhesion of the cover plates with the intermediate layer and providing the molded component with the optimum buckling stiffness.
A mixture of solid and fluid resin with a ratio of 100:20 to 100:10 and a hardener is suitable as the epoxy resin. A substance which minimizes the tendency of the filler material to flow at elevated temperatures, such as microdispersed silicic acid, should be admixed into the epoxy resin. It has also proven useful for reshaping by deep-drawing if the epoxy resin has microspheres made of glass, expanded clay, or aluminum admixed into it. These types of microspheres can be easily displaced or broken without impairing the contact of the slugs with the cover plates, because the fragments press themselves into the zinc coating typically provided on the cover plates or into the cover plates themselves. These types of microspheres should have a diameter of 70-130 xcexcm, particularly approximately 100 xcexcm, and a wall thickness of  less than 10 xcexcm, i.e. they should have a magnitude which is smaller by more than a power of 10 than the thickness of the intermediate layer.
Furthermore, it has been shown to be advantageous if the epoxy resin has a substance which improves the adhesion, such as talcum, and/or a substance which improves the cohesion, such as wollastonite, admixed into it. It is also advantageous if the epoxy resin has at least one corrosion inhibitor admixed into it.
The process portion of the objects mentioned above is achieved in that the filler material for the intermediate layer is applied as a paste on to a band serving as the first cover plate as it passes, and the slugs are pressed into the filler material, after it has been applied, via a band fed as the second cover plate into a welding gap formed by two roller electrodes, at least until the slugs contact the cover plates, and, during welding with the cover plates, the slugs are largely pressed flat.
This type of process has the advantage of continuous production of a semifinished product in which the cover plates are firmly bonded with one another at defined locations by the slugs. The composite material can then be cut into slabs or coiled up. A requirement for coiling is, of course, that the filler material moves along with the deformation into the coil. Insofar as the filler material consists, according to an embodiment of the invention, of a material which only polymerizes at enamel stoving temperatures, this is possible without problems.
It has been shown to be advantageous for the production process if the welding of the slugs with the cover plates in the welding gap is ended before the smallest interval of the welding gap is reached. The further pressing of the composite material together until the smallest interval has been reached then serves for subsequent shaping and allows any gases which have been produced during welding to be pressed out.
The application of the filler material for the intermediate layer is preferably performed by extruding onto the first cover plate, particularly onto a preheated band. The slugs can then be pressed into a filler material of this type until contact is made via the second cover plate without expenditure of a large amount of force.
Uniform distribution of the slugs onto the band can easily be achieved by simultaneously placing multiple slugs on the intermediate layer in one or more rows transverse to the movement direction of the band.
According to a further embodiment, further processing into finished products can be performed subsequently to the process for production of a composite material as a semifinished product. This is performed by cutting the composite material to size, reshaping the blanks into molded components, particularly through deep-drawing, and then subjecting them to a heat treatment, particularly a cathodic immersion enameling with stoving of the enamel. Before the stove enameling, the composite material can be reshaped without disadvantageous consequences for the epoxy resin and its adhesion to the cover plates. It only polymerizes, particularly cross-links, during the stove enameling, so that the molded component obtains its final buckling stiffness then.
Preferably, the blanks are cut with a laser beam or the intermediate layer is cured at the cut edges with the beam of a laser or a mercury vapor lamp. This, in addition to the effect of the thixotropizing additives on the filler material, impedes the epoxy resin from being pressed out between the cover plates during further processing.
The facility for performing the process described is characterized by two reels with the bands for the cover plates and a resistance welding device formed by two strip electrodes or, preferably, by two roller electrodes whose welding current can be switched on and off at intervals, as well as an application device for the filler material, positioned over the lower of the two bands fed to the welding gap formed by the roller electrodes positioned at a mutual distance, and a feeding device for the slugs, positioned downstream in the direction of movement of the band, whose cycle for releasing of the slugs is synchronized with the welding intervals.
The application device is preferably an extruder with a slot die. The feeding device for the slugs can be a stamping press with multiple embossing stamps and feeding tubes, positioned parallel to one another and transverse to the direction of movement of the band, which each eject a slug during every stamping stroke.